It is extremely rare for mortgages to be assumed but in very rare occasions it may make some sense, Interest rates today hover near all time lows with the average overnight rate on a 30 year fixed conforming note sitting this morning at 3.44% and a 15 year fixed conforming at 2.72%. Hard to beat todays rates and they are one of the main reasons the real estate markets is doing as well as it has been.
If there is no benefit to paying if off now and you feel it could be used to the advantage of a potential buyer keep it live and if the assumption doesn't work have the closing agent pay of the loan at that time.
Whenever doing anything real estate we advise everyone to interview at least three brokers. Make sure you find one who can answer your questions about how they will professionally assist you to meet your goal's and what it is going to cost you.
The very best of luck to you with all your real estate goals. Fins up.....